Wouldn't be the first time.

"Zelda for Wii U is coming in 2015."

"Zelda for Wii U is coming in 2016."

"The Nintendo 64 is coming in 1995."

Ehhm, yes they do.
From the first concepts to final release the standard time is about 5 years, give or take a few.
Yes, if you want/need to be faster that is quite possible, and yes, the first years are very slow with a small team as the new console isnt priority yet. But under normal circumstances you can assume that the work on a new console begins about 6 month after the release of the last. Only notable difference in the last decades was on of the old Xbox-models if I recall correctly.
This process can be accelerated, eg. put more people to it early on, dont experiment that much and specify the prototypes faster, etc...
but the balancing of features, tech, price, specs, software, os, games, etc... still takes a whole lot of time.
At the same time the whole thing must be communicated with third parties, you need to prepare marketing, and you have to take care of your current buisness, as I said a f*ckton of work that needs to be coordinated between a myriad of people and companies and subsidiaries and much more.

There are hundreds of reasons why a console is or can be postponed, same with games, especially if they are developed for new hardware, trust me, I actually work in game development, I know this stuff! Its fun yes, but it sure as hell aint easy!

Yeah, I don't buy the whole next idea thing. Last gen was different as both Sony and MS were outdone by Nintendo. Sony lost a lot of money and MS was the underdog who beat the King.

This gen both MS and Sony have beaten Nintendo and have already shown their hand for their next console that will be on the market for at least another 3 years from launch and that hand is 4K and VR.

Neither Sony or MS will stop production and go back to the drawing board just because Nintendo has chosen to go in a different direction. Even if Nintendo release it next year, the "idea" can still be taken for their following console if the "idea" is successful.

If this so called idea is going back to cartridges, MS and Sony won't bother as both are invested in their consoles supporting DVD and BluRay.

If their next idea is streaming, both MS and Sony have their own versions of cross platform play.

I can't think of what this idea is outside of Nintendo doing a Sony and going with them retro feels and releasing a cartridge powered system where the games can be played on the console and on a handheld. A console that will also allow you to play the DeNa games and sync your purchases across devices.

If Nintendo could launch this holiday season they would have, but they couldn't. It's no longer about worries on whether the other two will copy their "idea," but I think it's about ensuring there is content on the system at launch other than Zelda and the system is ready for release. If they don't announce it officially this year, I see another delay where we get a reveal at E3 with an early autumn release.

If in a 5 years you take 2 to 3 years at a very slow pace just throwing ideas and concept you can't really say the development takes 5 year... as I said, it start basically as soon as they release one console up to the release of the next one. Will you say then that Xbox One took 10 years to be developed? Nope... it took they 2-3 years and the others were just concepts and technology assessment.

concepting, technology assessment and working that into a prototype is what one usually refers to as "developing a console"

The 2-3 years you are talking about are the home-stretch, finalizing the tech, making sure everythings ready at launch, prepping pr, etc...
Its the last part of the development as a whole. Again, I know how easy it is to underestimate the time stuff like this takes, but seriously, this is, just like developing a game, a whole lot more work than one might believe and it often takes much longer than one would expect.

Man, I also deal with high technology and I'm certain that if my company really had to take 5 years to just implement an evolution in a system they would be out of bussinees... of course the project as a whole could even take some decades but a P&D can't be full going 5 years per product and expect a good profitability.